Fizzy Distortion in Background of new Push Pull 6V6 Build

[eurekaiv]

I just wrapped up the electrical work on a tweed deluxe-ish build I P2P wired into an old Voycall intercom amp I picked up on eBay. The build is based on a schematic I found for ÷13 CJ11 but with a mid control since I had space for 5 knobs. It's basically a tweaked blackface front end on what appears to be a tweed deluxe-like back end. My build is working well in terms of functionality after tracking down a wiring mistake in my master volume implementation and it's dead quiet despite the P2P wiring and non-ideal layout (I basically "made it fit" it into the chassis with the tube sockets and transformers I had) but there's a bit of a fizzly background distortion that trails notes and I'm not sure where this is coming from or if it's just normal and my ears caught it and won't let go. I'm really not sure what to call this. I can try and add some clips but I doubt they come out great with just my phone. For those interested... with a GZ34 I get about 22V across the cathode resistor (321Ω) and around 377V on the plates which gives me about 12W dissipation if the calculations are correct.

[tubeswell]

... with a GZ34 I get about 22V across the cathode resistor (321Ω) and around 377V on the plates which gives me about 12W dissipation if the calculations are correct.

22V/321R = 68.54mA (minus about 4mA screen current for both 6V6s = 64.5mA or 32.25mA per tube)


377-22 = 355V plate-to-cathode


355V x 0.0323A = 11.5W


you could probably just use standard 250R cathode resistor


Re the fizziness - did you swap the pre-amp tubes (to see if it was a tube)?

[eurekaiv]

I swapped the 12AX7 positions but didn’t change them out. I’ll try that and see if anything changes. The swap resulted in the amp sounding a bit different but the distortion didn’t change. I’ll post some pics in case anyone wants to see the rats nest.

As an aside... is there a brand of carbon film resistor with a beefier lead? It seems like everything I buy from mouser is like an angels hair compared to the old carbon comp stuff of yesteryear.

[tubeswell]

First thing to try is fresh tubes.

[eurekaiv]

I have a lot of tubes lying around but none of them are what I would call "fresh." They're all NOS or used pulls. I just went through 4 or 5 old RCA red and white label 12AX7s and even tried a nice triple mica GE 12AU7 (in the preamp slot) and all were the same. For 6V6s I'll have to dig some more or start pulling them out of other amps. Thought maybe the speaker too but the little champ I built last week in an old organ chassis does not do this, it sounds perfectly fine. I made a clip, gonna try and put it on youtube to share if it's audible enough.

Pics attached... knobs need to be replaced since they were spray painted and didn't clean up all that well. I also added lighting inside the chassis as I thought it might look fun with the all the vent holes around the power tube sockets. I think I might switch to amber though.

[eurekaiv]

You can hear it pretty well in the video... it’s a fizzle at the end of notes that sorta gates out and sputters a bit.

[tubeswell]

...I just went through 4 or 5 old RCA red and white label 12AX7s and even tried a nice triple mica GE 12AU7 (in the preamp slot) and all were the same. ...

Pics attached...

In that case, seeing as how you've still got the fizz with different pre-amp tubes, and seeing the pic's you've posted, I'm inclined to say that the fizzy distortion you report is most probably lead dress. I notice a bunch of leads everywhere, some of them criss-cross, some of then are next to and long-side each other.


Treat each signal lead  (be it to a pot wiper-to-grid, or input to grid, or coupling-cap-to-grid) and each pre-amp tube grid pin, and each resistor lead and cap lead that are in the signal path, as an inductor*. And then dress your other leads to keep higher-current or higher voltage leads at least 1" away from these inductors.

*Remember that with electro-magnetism, changing current(over time) within a wire, results in corresponding change in electro-motive force within the electro-magnetic field that is circumferentially surrounding the wire, and these electro-motive fields can induce a further corresponding change in current in other nearby wires. And that's one source of oscillation and/or cross-talk.

[eurekaiv]

Thans for the tip. There’s only a couple places where the signal path comes particualrly close to higher voltages (other than at the tube pins themselves) so hopefully I can find something there.

[pdf64]

You can hear it pretty well in the video... it’s a fizzle at the end of notes that sorta gates out and sputters a bit.

I can’t really make out what you’re referring to, the next note gets picked in short order.
I’d use a constant tone to try and provoke the issue.
Have you eliminated it being due to speaker or cab?

[eurekaiv]

There’s a little distorted fizzle behind the picked note. It goes away quickly, which is why I pick another note quickly. The amp is set almost totally clean in the vid so there really shouldn’t be any distortion at all. I’m going to try and look at it on the scope today, to see if I can see it, and if so, see if I can effect any change with tubes or lead dress. I really need to swap 6V6s too but that’ll be trickier since it’s unlikely I have another pair that match up well out of the 4 or 5 spares I have.

[fossilshark]

I had an issue with my amplifiers where 1N4007 rectifier diodes didn't switch fast enough (I think it was a bit more complex then that) causing what you might be describing. Consider swapping your rectifier diodes for SR2010.

[eurekaiv]

Thanks for the suggestion. I'm using a GZ34 in this so no diodes. I did try a 5U4 but it was the same.

[pdf64]

Is that the output transformer, inside the chassis, right by the input jacks?

[eurekaiv]

Yeah... OT is inside the chassis near the inputs. Not the best idea but it's how the original amp was laid out so I did my best to not make extra work by moving it. I know it looks a mess but I did route signals away from HV for the most part. A couple are kinda close, mainly at the output of the phase inverter, but the focal length of the image makes most look a lot closer than they are. I have a couple old P2P amps that are WAY worse. That of course doesn't make it ok, but those amp don't exhibit this strange clipping behavior either.

Anyway, I did find an issue...  the top half of the wave form is clipping off beginning at the output of the second stage. The input (after the volume control) gives me a perfect sine wave. Voltages look ok on both sides of the 12AX7. I tried chopsticking some leads around and see if I could incite some change but this didn't seem to help. I disconnected the "boost" switch around the cathode cap and I might try and remove the 100K resistor there to see if it doesn't like this mod. Otherwise, my next thought was to disconnect the next stage and see if maybe something there is causing the problem backwards.

[shooter]

I disconnected the "boost" switch

try lifting C8, should drop gain so the clip is gone.

are you shooting for pure clean from the pre?

[eurekaiv]

It’s definitely not a goal to have a clean preamp but I do think an amp like this should be able to reproduce a clean sine wave at low volumes.

[shooter]

at low volumes.

what was the voltage you injected?
I typically use ~150mVrms
where was the volume pot set?

[eurekaiv]

Good question, I forgot to even check. I usually keep it set about 1V P2P on the generator (so ~300mVrms I think) but I was messing with that a bit recently while fiddling with a ss stereo amp so it might be too hot.

[eurekaiv]

Shooter nailed this one... my generator was at about 2V p2p. The preamp looks fine now. It still sounds pretty bad but waves look fine all the way to the output with the volume low enough for a clean sine. I now suspect the output transformer might have to be junked. I have a couple pulled from some old organ amps that should work to at least test this theory. Not gonna be much fun cobbling those into this thing though. 

[pdf64]

Why is the OT suspect?
I suggest to try bodging in a screening plate between the input jacks and OT.

[shooter]

+1
don't jump off bridges before you check for water 

with your clean sine, scope the speaker, dial for MAX clean (amp knobs)
if good, does your sig-gen do 2 tones?, start there

move to guitar and try hard to get 1 note, watch the "ringing"

[eurekaiv]

Sometimes you just act on a hunch... it actually wasn't bad at all to tack it in. Ugly little transformer, short little leads and about 7800k into only 8Ω. Sounded pretty much exactly the same. Oh well. Putting it aside for the day, maybe two. We'll see... I went and found a couple CJ11 clips on youtube though, and I think I hear the same thing in those. I'm starting to get the impression this is just how this design starts clipping. When dialed to full clean it's def clean. The first signs of clipping sound like this little fizzle on top of the clean sound and then that fizzle starts to blend in more with more overdrive at more frequencies as you turn it up.

[tubeswell]

Anyway, I did find an issue...  the top half of the wave form is clipping off beginning at the output of the second stage. The input (after the volume control) gives me a perfect sine wave.

Why have you got a 47k resistor to ground from the grid of v2a? (This would be heavily loading the output signal from V1B). Suggest you make this 250k minimum (or 470k)


(If you want to attenuate the signal to V2a, then use a proper voltage divider between the coupling cap and the grid of V2a)

[eurekaiv]

The amp this is a basic copy of has a 47K there. What you say makes perfect sense to me but I just went with the schematic I found (which I redrew for myself) and photos of the guts clearly show this as well. You can see the 47k to ground and a piece of shielded cable connected to it.

Not only will this attenuate the signal isn't it also a high pass filter? Looks like it will cutoff around 1.5k. The topic on FSB where I found the original amp design drawings mentioned the Trainwreck Liverpool having a similar grid resistor to this and that preamp design looks kinda similar except it uses a much smaller value coupling cap which makes for a much different filtering if I understand correctly.

[ac427v]

I think you have ultrasonic oscillation caused by too much gain at higher volumes. My favorite build uses this circuit. But no bypass cap on v1b or v2a. V2-a has a 1.5k cathode resistor. I copied the 47k load resistor in the original circuit as you did.

[eurekaiv]

Whatever it is, it’s coming from the phase inverter or power section. I lifted C12 (coupling cap to V2B) and alligator clipped it into the power section of a Champ I built and it sounded perfect.

[shooter]

try tacking a 500pF - .001uF across (in parallel) with the PI plate R


"fix" for what ac427v mentioned

[eurekaiv]

I'll give it a shot. I'm pretty sure it's coming from the phase inverter. I probed a few spots with the scope while playing through the amp... I'm getting little peaks that show up and linger a bit on one half of the waveform when looking where the .1 coupling caps meat the master volume. I don't get this at the input to the phase inverter.

[shooter]

I'm getting little peaks that show up

If they show up while poking, I'd look at the solder
if they show up random and un-announced, cap 'em 

[jim]

Trailing fizz on a note could be a connection you forgot to solder or perhaps a cold joint. Look closely.   Might not be that complicated......Jim

[eurekaiv]

I tried a 500pf cap and it made no discernible difference to me. Probed around for missed solder joints again and I'm confident there are none.

[HotBluePlates]

... there's a bit of a fizzly background distortion that trails notes ...

Normally I'd look to cold solder joints or imperfect connections as a potential source for this.

Loose tube sockets?

The build seems to rely a lot on terminal strips bolted to the chassis for grounding.  Yet the chassis appears to have a coat of hammertone paint on the inside.  Did you scrape away the paint at all the ground points?

[brewdude]

You might try a grid stopper before the cathodyne PI.

[brewdude]

Is it uncommon to put a PPIMV on a cathodyne PI? 

[pdf64]

Is it uncommon to put a PPIMV on a cathodyne PI? 

I can’t envisage a valid rationale for doing so 

[eurekaiv]

Loose tock sockets?

The build seems to rely a lot on terminal strips bolted to the chassis for grounding.  Yet the chassis appears to have a coat of hammertone paint on the inside.  Did you scrape away the paint at all the ground points?

None of the terminal strips I ran the ground bus on are connected to ground. The star grounding point is on a lug mounted to one of the transformer bolts and I scraped away the paint to the chassis underneath. The ground there is very solid. One of the thing on my list of things to do before calling this complete was soldering the lug to the chassis but it’s fine for now.

I built this amp with the master volume because it was about the only commerical (and so I would expect soundly designed) layout I could find that met the specs of what I was after based on the chassis and iron I had... 2 6V6s, 2 12AX7s, a tube rectifier and room for volume, tmb and a master volume on the front panel. I like master volumes in general so having one in a higher power build was def a necessity for me. I can’t play loud guitar where I live and even single ended champ cranked up is a bit much anytime other then maybe a few minutes on a weekend afternoon.

[brewdude]

I think I would be inclined to move the MV to between V1b and V2b.
I would definitely add a grid stopper before the cathodyne PI.

And, I would suggest that you look into a VVR. 

[eurekaiv]

I discovered my issue, and as usual, it's one of those "I'm a giant nitwit" things I overlooked. I'm very frequently overlooking obvious things right in front of my face while having little trouble with the complicated stuff. In any case, while I was careful to make sure that my additional lighting was not mounted directly to the chassis so the bayonet sockets were attached to a piece of fiberboard to isolate them, I forgot that screws protrude through the chassis and would obviously make contact to ground there. So the amp started sounding much better once I addressed that huge biff. I did some further mods and I'm super happy with how it sounds now so I'll post those and my final schematic soon. I'm sure not many of you will care but it can't hurt to share.

Also, I know the layout looks rough but it's dead quiet. It's by far the quietest scratch built amp I've point to point wired so far but the others are all single ended 6V6 or el84 based.

[shooter]

frequently overlooking obvious things

yep, walk into a tree while watching a bird 


SE can't cheap like PP amps, so you gotta get the design right, there's lots to steal from

[bmccowan]

Great!

I'm sure not many of you will care but it can't hurt to share

I think a lot of us care, and love to look at the schematics of successful projects. I'm always disappointed when folks don't follow-up with project results.
And Shooter, I have walked into a tree while watching a bird, of sorts. Downtown Portland Maine and the drivers on the street were so amused they started honking their appreciation for my folly.

[eurekaiv]

I'm calling this one done. Final schematic below and notes of the mods/selections I made based on feedback in this thread and using my ears.

1. I added a 4.7k resistor to ground off the mid control. Basically, it's very scooped even with the 4.7k but I get far more usable range out of the pot this way. In hindsight, something like a 7.5k resistor and 10k pot might have given the best usable range. With the 25k pot there's almost no audible difference in the tone at the last 1/4 turn. I didn't feel like messing with it anymore though.

2. I removed the cap from V2A's cathode to lower the gain at this stage.

3. Switched this to a fixed bias cathodyne phase inverter (aka Paul C mod) and thought it changed the amp's character slightly for the better. Maybe a little less sizzle on top?

4. I changed the cap at V1A's cathode to a 2.2uf. I generally set an amp fairly clean but this setup allows for a tighter/crunchier overdrive sound without the boost pulled and then with the boost pulled, it fattens things up a lot. In use, there's enough gain on the high input to get an nice classic rock rhythm tone without the boost pulled so you can really dial in nice variety of fatter or tighter tones by varying the volume control with/without the boost engaged. Pretty neat IMO. The guitar I used to dial it in had a PAF clone bridge pickup FWIW, so not crazy hot.

5. I added a 3 way NFB switch with 10k and 22k options but I probably will only use 10k as I like it's sound the best. I tried this at a both V1A and B and it sounded best at V1A. Couldn't tell you why. This also tightens up the sound a bit but isn't really lowering the amount of drive available. I actually liked about 8k here but I didn't have an 8k resistor and there wasn't all that much of a difference between 8 and 10 so didn't bother to double one up to get there. Much less than 8k and the amp starts shutting off. I guess I like a lot of negative feedback in this circuit.

6. I clipped in some pots in for the mixing resistor at V2A and the grid resistor at V2B and settled on 100K and 470K respectively, as noticeably the best sounding values here in these positions.

Other notes... this was built using the iron/chassis/cage from a Lake Manufacturing Co. VOYCALL 14A intercom amplifier. Final voltage on the 6V6 plates is about 370. I think I measured the power transformer at 560V AC before I started putting this together but I don't really remember. So it's maybe a 275-0-275? Or something close to there. Output transformer was 7380Ω into 8Ω. It also has a 4 and 2Ω tap. Wish it had a 16 but ah well.

I managed to find a couple Harmon Kardon branded 6V6s in my stash that were within 1ma of each other. So that's nice I guess. Pretty sure they're Sylvania made as I have a single Sylvania 6V6 that looks pretty much identical.

[PRR]

> 2x6V6_VTMBMV.png

R18 is probably not 1.5K.

[eurekaiv]

R18 is definitely not 1.5K. 2.2M is the correct value. I’ll update.

[JB]

In any case, while I was careful to make sure that my additional lighting was not mounted directly to the chassis so the bayonet sockets were attached to a piece of fiberboard to isolate them, I forgot that screws protrude through the chassis and would obviously make contact to ground there.

My 5E3 has a little bit of trailing fizz so I'm curious to know exactly what happend here, but not sure I follow.  "Additional Lighting"?  Are you saying a ground point on a board was touching the chassis when it shouldn't have been, and hence you had a ground loop?

[ac427v]

I've never seen negative feedback from the output transformer connected to the cathode of the first gainstage. Seems like it would act to correct any distortion in the entire amp. How does it work in real life?

[HotBluePlates]

I discovered my issue ... my additional lighting was not mounted directly to the chassis so the bayonet sockets were attached to a piece of fiberboard to isolate them, I forgot that screws protrude through the chassis and would obviously make contact to ground there. So the amp started sounding much better once I addressed that ...

... not sure I follow.  "Additional Lighting"?  Are you saying a ground point on a board was touching the chassis when it shouldn't have been, and hence you had a ground loop?

Same here: I'm curious what fixed your problem.

I've never seen negative feedback from the output transformer connected to the cathode of the first gainstage. ... How does it work in real life?

I would be more concerned about how much phase shift is enclosed in that very large loop.  That can cause the amp to oscillate.  Which then makes me wonder if the buzzing was audible in one setting of the feedback switch, but not another.   

[eurekaiv]

In any case, while I was careful to make sure that my additional lighting was not mounted directly to the chassis so the bayonet sockets were attached to a piece of fiberboard to isolate them, I forgot that screws protrude through the chassis and would obviously make contact to ground there.

My 5E3 has a little bit of trailing fizz so I'm curious to know exactly what happend here, but not sure I follow.  "Additional Lighting"?  Are you saying a ground point on a board was touching the chassis when it shouldn't have been, and hence you had a ground loop?

I ran a couple extra led bulbs off the pilot light/filament line and the bayonet lamp sockets I used (pulled from an old pinball machine) needed to be isolated from the chassis. Unfortunately, I didn’t isolate them properly the first time. Then I decided I didn’t like them anyway and just removed them.

[eurekaiv]

I would be more concerned about how much phase shift is enclosed in that very large loop.  That can cause the amp to oscillate.  Which then makes me wonder if the buzzing was audible in one setting of the feedback switch, but not another.   

I have no idea about the negative feedback, I just tried it with a pot and did what I thought sounded best. I’m not getting any noticeable oscillation even with the boost maxed. This was the last mod I made. Where would you stick it? I heard very little effect attached to the tone recovery stage which may have something to do with the boost mod that’s already there (from the original design inspiration).

I think most of the fizzy “bad sound” I was getting was due to some sort of oscillation from having the 6v filament on the chassis ground, due to improperly wiring the additional pilot lights.

[shooter]

here's a good primer on NFB;


http://www.aikenamps.com/index.php/what-is-negative-feedback
get a cup of coffee, read slow 

[HotBluePlates]

I would be more concerned about how much phase shift is enclosed in that very large loop.  That can cause the amp to oscillate.  ...

... I’m not getting any noticeable oscillation even with the boost maxed. ... Where would you stick it? ...

I'll defer to PRR and perhaps pdf64; I'm no great Feedback Thinker & instead copy what I've seen that works.  Typically, I'd use it from speaker to the phase inverter or driver stage (for a cathodyne/split-load inverter, it's the gain stage immediately before the inverter).

RDH4 and other old books on feedback warn there's a balance that happens between "large amount of feedback" and "phase shift inside the loop" that can cause negative feedback to be turned into positive feedback.

I never dug too deeply into the concept, as I never really wanted more than the small amount guitar amps typically use.